Plate Tectonics: GPS Data, Boundary Zones, and Earthquake Hazards

Christopher Berg, Orange Coast College
Julie Elliott, Purdue University
Beth Pratt-Sitaula, UNAVCO


Summary

Students work with high precision GPS data to explore how motion near a plate boundary is distributed over a larger region and hypothesize the area over which boundary-related earthquake hazards might exist. Primary emphasis is strike-slip regions (examples from Alaska, Dominican Republic, and California).

Motivating Question: Is all motion on the plate boundary? How far away from the plate boundary might earthquakes occur?

Strengths of Module

  • Earthquakes are an engaging entry point for students to learn more about plate tectonics and plate boundaries.
  • Most students carry at least one GPS receiver around with them all the time in their phones, so this provides a good entry point for students to relate to the data in the module.
  • Scaffolds quantitative skills on creating/interpreting graphs, adding vectors, and plotting on a map, and then asks students to draw on these quantitative findings to reason an answer to the overarching question.
  • Noise and variability in the data are potentially intriguing without being overwhelming.
  • Students are able to choose from hundreds of GPS stations to decide the ones they will use.

Learning Goals

Students are able to:

  1. Calculate horizontal rates of change from GPS positions (either graphically OR from spreadsheet data) and plot the vectors on a map
  2. Select stations in a transect across a strike-slip fault, download the data, and determine the horizontal rates for each, and plot
  3. Interpret the changes in motion across the transect and estimate the width of area vulnerable to earthquakes from the plate boundary activity

Context for Use

This module was intended for introductory earth science students at the college or secondary level. It also works well for earlier-stage geology majors. Appropriate courses include: physical geology, natural hazards, plate tectonics, and structural geology. Data analysis works best if students have computers (individually or in small groups), although the module can also be done with provided graphs (which the instructor would have to print) instead of downloaded data, from which the students graph the data themselves in a spreadsheet program (ex. Excel or Sheets).

Description and Teaching Materials

Why this Matters:

This module gives students the chance to investigate where earthquakes might occur, while learning valuable analysis skills with real data.

Quick outline/overview of the activities in this module

  • Activity A: Understand the horizontal motion from just one GPS station
  • Activity B: Compare GPS data from pairs of stations near two different strike-slip regions (Alaska and Dominican Republic)
  • Activity C: Students choose their own transect of 4-5 stations in California and eastern Nevada to look at how motions change away from the "plate boundary line" on the map.
  • (Opt.) Activity D: Short extension to discuss velocities across a wider area of the western US and/or other applications of GPS for plate tectonic and earthquake issues.

The first decision you will need to make as an instructor is whether your students will download the .csv files and graph the position data themselves in a spreadsheet program and calculate rates OR if you will have them download (or you provide them) time series graphs, from which they graphically determine rates. The exercise is written assuming the students download the .csv data themselves, but it can be easily modified to use graphs instead. We highly recommend that you work through the exercise as a student would, prior to starting the module.

Activity A

Before digging into the data analysis, you will need to present at least some parts of the provided presentation, unless students have learned about earthquakes, GPS reference frames, and/or GPS data elsewhere in the course. After the background presentation, students download data from a GPS station, calculate North and East rates from GPS position data, determine the overall horizontal motion vector, and plot it on a map. The suggested station to use is from coastal Washington State. This is the only station in the module that is not from a strike-slip environment, but the main purpose of the activity is to learn the basic GPS data analysis skills and not to discuss the tectonic setting. We choose this station so that the module gives students practice calculating and plotting vectors going in different directions. You could, however, choose any station you prefer them to use from UNAVCO's GPS Network Monitoring page. Although UNAVCO does not consider this the primary data portal, it is the most intuitive entry point for non-geodesists and many professional geologists use it.

This activity is probably best done individually if enough computers are available, so each student has to gain the skills for themselves.

Activity B

Students look at GPS data from two stations along a strike-slip fault. Half the class looks at the Denali Fault, AK, and half at a fault in the Caribbean (Dominican Republic). They calculate the horizontal motion at their two stations and plot the vectors on a map. Next, there is a class discussion about the findings in relation to strike-slip faults and where the earthquake hazard is. What the students will find is that, in the North American reference frame, the offset motion in Alaska is the "classic" perception of a strike-slip fault with one side of the fault going west and the other going east. In Dominican Republic, both stations show eastward motion but at different rates. The difference between a slower station and faster station shows that strain is accumulating between them and earthquakes are possible. Understanding that both of these velocity fields can indicate a strike-slip fault is challenging for students, but necessary to do Activity C. For this exercise, we provide (rather than have students download) the data since 2015. Both areas experienced earthquakes prior to that, which greatly complicate the record. If you wish to give them the full time series from which to clip an appropriate (steady-state) post-earthquake rate, the links to the station pages are in the provided spreadsheet.

This activity can be done in small groups or individually.

Activity C

This is the activity in which the students really work to address the motivating question of how far away from the "boundary line" plate boundary-related earthquakes can occur. Students will build upon skills from Activities A & B to interpret GPS time-series from 4-5 locations along a southwest-to-northeast transect in California and eastern Nevada (selected by the students). They use these data to assess regional patterns and tie them to the tectonic setting. Students will compare station motions and estimate how far from the main plate boundary the motion/earthquake hazard extends. Students will need to locate stations on a map, calculate and plot motion vectors for each location, and communicate their results. They need to use quantitative reasoning to support their assertion of the plate boundary deformation area and likely earthquake hazard area.

This activity can be done in small groups or individually. It is best to plan on some final wrap-up discussion at the end of class or at later date if the final completion of Activity C is done as homework.

UNAVCO's GPS Network Monitoring page will be used as the data access point. Although UNAVCO does not consider this the primary data portal, it is the most intuitive entry point for non-geodesists.

(Opt.) Activity D

If you are interested in giving students a more comprehensive view of the GPS-measured motions across the western US, you could show them one of these images and engage them in a short discussion of what else they notice on the map. OR you could show them an animation to extend discussion to other tectonic settings or GPS applications.

Teaching Materials:

Teaching Notes and Tips

  • The Teaching with EDDIE page has some great general suggestions for teaching with real data.
  • The Network of the America's (NOTA) stations pages (ex. P402 in Activity A).
  • A number of GPS time series have irregularities that students might ask about. Ideally, you will engage the students in noticing the irregularities and considering why they might be there. Some things you might see in GPS time series:
    • Gaps - equipment malfunction is a common one, but vandalism, wildlife, and lightning strikes can also cause gaps; very remote stations may have to wait months before a field engineer can visit them to address the issue.
    • Seasonal cyclicity or deflections in the time series - the majority of the seasonal signal is seen in the GPS vertical position (not investigated in this module) as changes in groundwater or snow "weight" and "unweight" the ground. In some stations, this the annual water/ice signal may cause horizontal motion too -- for example if a station is at the edge of a valley that gets a big seasonal influx of water the station might "tip" towards the valley center during that time of the year. A multi-year event such a drought could cause a longer change.
    • Offsets - two of the most common reasons for these are earthquakes or equipment changes. If a student is curious about an offset they see, encourage them to look at a nearby station; if that station shows a similar feature, an earthquake is a likely cause (also they could check the IRIS Earthquake Browser). If no earthquake is apparent, then perhaps an equipment change such as an antenna replacement occurred.
    • Extra scatter of data points ("noisiness") - not always clear what this is from but possibilities include snow or temporary/partial blocking of some of the sky view by a large object
  • Do know that GPS data are dynamic in nature, and earthquakes and other processes can change the vectors observed from one time to the next.
  • A great resource for supporting students' math skill growth is The Math You Need When You Need It. For instance, if you are having your students determine rates graphically, you may want to have them do the unit Constructing a best fit line.
  • Definitely work through the student activity before trying to do it with your class.
  • Increasingly, students seem to be relaying on Google Sheets instead of Excel. You may want to test drive the data analysis process in both software.
  • It can also be well worth your while to work with students on using Help within their software of interest in order to help them learn how to find answers for themselves rather than wait for you to come around the room. It takes a bit of extra time at the start but they gain the valuable skill of searching for solutions.
  • GPS data files also show vertical motion, which is very interesting too, but more than the scope of this module. If you are interested in knowing/teaching more about the vertical signal, see some of the References below.

Measures of Student Success

Activities A & B will have opportunities for formative assessments from evaluating student abilities to interpret time-series data. Group discussions will provide opportunities for additional informal assessment. Activity C will require students to demonstrate their ability to analyze and interpret data and communicate their results and serves as a summative assessment for the module. A rubric is provided for the most summative questions of Activity C.

References and Resources